You know how it's absolutely not OK to date your mate's ex? Well, most Bundesliga clubs have their own unwritten rule that reinforces their love for their fans: never assign the No.12 shirt to a player.
Bayern Munich and Werder Bremen are among those to have officially retired the jersey in honour of the fabled '12th man' (the supporters), while all but two of the 18 current Bundesliga sides - Hertha Berlin and Wolfsburg being the exceptions - have refrained from using it at all in 2018/19.
It's not in the rules - numbers 1-40 have all been fair game for over two decades, with No.1 reserved exclusively for goalkeepers - but the odds of you ever seeing the No.12 on the back of a Bundesliga shirt are about as slim as a swarm of lesser-spotted woodpeckers descending on the Signal Iduna Park on matchday (no offence to Hertha and Wolfsburg reserve goalkeepers, Dennis Smarsch and Pavao Pervan!).
For outfield players, at least, Nummer Zwölf appears to be off-limits.
So what does that tell us? No Bundesliga player - no matter how decorated, or superstitious - is bigger than a club's fans. Because when the chips are down, the supporters are the ones with the collective power to move mountains.
How else do you think Borussia Dortmund were able to come from 1-0 and 2-1 down to beat record champions Bayern 3-2 in Der Klassiker recently? The result might have panned out differently without the fervent backing of 75,000 or so black-yellow-clad No.12s.
But then this is the Bundesliga, where the highest average attendances in world football combined with some of the lowest ticket prices create the most electrifying fan experience on the planet. The supporters could be watching paint dry, and they'd still turn it into a spectacle.
It's football as it's meant to be. And the 12th man is king. They've got the T-shirt to prove it.